The latest trend in the development of automotive foam parts is integrated molded seats which is expected to improve productivity and significantly reduce labor costs. These seats are usually produced by pouring urethane foaming liquid mixture onto a surface covering material, e.g., cloth, vinyl, etc., prepositioned in a mold. The mold is then closed and the foaming mixture is allowed to expand and fill the mold cavity such that the resulting product is an integrated seat composed of a surface covering material and a urethane foam core.
The integrated foam parts sometimes have surface defects of small voids, e.g., in a volume between 1 to 10 cubic centimeters. These voids become a cause of rejection for the integrated foam parts which result in higher production costs. Void filling by means of a mini-scale foaming with urethane foams could significantly reduce the production costs. By mini-scale foaming, we mean a foaming process in which only a small amount of foaming reactants are mixed together to form a volume of foam less than 10 cubic centimeters.
Existing techniques of producing urethane foams have many drawbacks when used in mini-scale foaming. These drawbacks include higher compression set values of the foam produced and formation of hard spots by the penetration of foaming liquid into the existing foam cells. This invention provides a novel method to solve these problems by employing specific polyols and specific thixotropic agents.
It is known that polyurethane foams are prepared by the reaction of a polyisocyanate with a polyol in the presence of a blowing agent, a surfactant and a catalyst. It is also known that high resilience (HR) urethane foams are prepared by using a graft polyol or a urea-dispersion polyol as the polyol component and toluene diisocyanate (TDI), diphenylmethane diisocyanate (MDI), or a mixture of TDI and MDI as the polyisocyanate component. Such HR foams have been used for automotive seat cushions and furniture cushions.
Conventional methods of producing automotive seats have been composed of labor-intensive processes, and therefore, reduction of its production costs were difficult. A latest trend in automotive seat production is the one step process for producing automotive seats. This is called an integrated molded seat process. Due to its one step process, it can save labor costs and significantly increase productivity.
The process is composed of the following operating steps: a cloth or vinyl material as a surface covering material is prepositioned on the inner surface of a mold; a urethane foaming mixture is poured onto the surface of the cloth in the mold and the mold lid is immediately closed; the foam is allowed to expand in order to fill the mold cavity. Seat cushions frequently have a concave surface, and therefore, air voids have the tendency to form on the surface of the molded part.
Attempts have been made to use existing urethane foam technologies to solve the problem of air voids in integrated molded foam seats. However, such attempts have resulted in no success due to several reasons. First, the void filling is a mini-scale foaming and the exothermic reaction heat dissipates faster than desired. As a result, the foams produced are not completely cured and have a high compression set. Secondly, the conventional urethane-foaming reactants are in a low viscosity liquid state and, therefore, penetrate easily into existing foam cells, which then become hard spots. Finally, in mini-scale foaming, where the total volume of foam is small, even a minute variation in the quantity of the components mixed can significantly change their ratio and further affect the compression set property.
It is therefore an object of the present invention to provide a method of making small volume of urethane foam which has high resiliency and low compression set.
It is another object of the present invention to provide a method of making small volume of urethane foam in which the properties of the foam produced is more tolerable to errors in the isocyanate/polyol mixing ratios.
It is yet another object of the present invention to provide a method of making small volume of urethane foam in which the viscosity of the reactants is sufficiently high such that the reactants cannot penetrate into existing foam cells when said reactants are injected into an air void situated in an integrated foam part.